Evaluation of parasite resistance in a beef cattle operation with use of extended-release eprinomectin for 3 years

Objective: Our objective was to determine whether parasite resistance to extended-release eprinomectin was present in an operation after 3 yr of eprinomectin use. Materials and Methods: Fall-born Angus × Simmental heifers (224 ± 22 d of age; 171 ± 17.8 kg initial BW) were stratified by d −2 fecal egg count (FEC) and BW and assigned to 1 of 9 groups (7 heifers per group). Groups were then assigned to 1 of 3 treatments: extended-release eprinomectin (ERE; n = 3), extended-release eprinomectin and oxfendazole (ERE+O; n = 3), or saline control (CON; n = 3). Throughout the experiment, FEC and packed cell volume were evaluated 6 times, and a fecal egg reduction test was conducted on d 28. Results and Discussion: There was a treatment × time interaction (P < 0.01) for FEC as FEC was not different on d −2 and 28. However, CON had greater FEC than ERE and ERE+O on d 55 and 83. Additionally, at d 55 ERE had greater FEC than ERE+O. On d 112 and 167, FEC was similar among all treatments. Fecal egg reduction for ERE and ERE+O were above the 90% threshold (91 and 98% reduction, respectively). There was no treatment × time interaction or treatment effect (P ≥ 0.14) for packed cell volume. Implications and Applications: Based on the fecal egg reduction test (>90%), parasite resistance was not present in an operation after 3 yr of use of extended-release eprinomectin.